In the production of optical fibers, a resin coating is applied immediately after drawing of the glass fibers for protection and reinforcement of the glass fiber. Generally, two coatings are applied, a soft primary coating layer of a flexible resin (low modulus and low Tg) which is coated directly on the glass surface and a secondary coating layer of a rigid resin (higher modulus and higher Tg) which is provided over the primary coating layer. Often, the fibers for identification purposes will be further coated with an ink, which is a curable resin comprising a colorant (such as a pigment and/or a dye), or the secondary coating may be a colored secondary coating (i.e, comprise a colorant).
Several coated (and optionally inked) optical fibers can be bundled together to form a so-called optical fiber ribbon, e.g., four or eight coated (and optionally inked) optical fibers are arranged on a plane and secured with a binder to produce a ribbon structure having a rectangular cross section. Said binder material for binding several optical fibers to produce the optical fiber ribbon structure is called a ribbon matrix material. In addition, a material for the further binding of several optical fiber ribbons to produce multi-core optical fiber ribbons is called a bundling material.
One of the most important characteristics required nowadays for curable resins used as coating materials (for protective or identification purposes) for optical fibers is to have a cure speed that is sufficiently high to be applicable at the currently used increasing optical fiber drawing speeds while still being cured thoroughly. Moreover, this improved cure speed should be obtained without sacrificing the chemical and mechanical properties of the cured coating. At present, in the production of optical fibers and optical fiber assemblies, one of the limitations on how fast the production line can be operated is the cure speed of the coatings and/or binders. Accordingly, it is desirable to develop coatings and/or binders with faster cure speed.
Besides having a high cure speed, the coating must also fulfill many other requirements, in particular: exhibiting very little physical change over a long period of time and also over wide temperature ranges; having acceptable resistance to heat and light (and thus, showing acceptable aging properties such as a low degree of yellowing), to hydrolysis, to oil, and to chemicals such as acids and alkalis; absorbing only a relatively small amount of moisture and water; producing little hydrogen gas which adversely affects optical fibers; and the like.
Resins that cure on exposure to radiation such as ultraviolet radiation are favored in the industry, due to their fast cure, enabling the coated fiber to be produced at high speed. In many of these radiation curable resin compositions, use is made of urethane oligomers having reactive terminal groups (such as an acrylate or methacrylate functionality, herein referred to as (meth)acrylate functionality) and a polymer backbone. Generally, these compositions may further comprise reactive diluents, photoinitiators, and optionally suitable additives.
From WO 98/00351 it is known to use a mixture of two hydrogen abstraction free-radical photoinitiators and one α-cleavage, homolytic free-radical photoinitiator, in particular a mixture of 1.5 wt. % benzophenone, 0.5 wt. % benzil with sensitizer diethyl amine and 0.7 wt. % Irgacure 651.
Photoinitiator packages are commercially available e.g. Esacure KTO 46 (consisting of a benzophenone derivative, a phosphine oxide type photoinitiator and an oligomeric α-hydroxy acetophenone) and Esacure KIP 100 F that is a mixture of 70% of an oligomeric α-hydroxy acetophenone and 30% of dimethyl hydroxy acetophenone.
So far, however, the known radiation-curable compositions also if they comprise a combination of photoinitiators in varying amounts do not provide satisfying properties with respect to cure speed in combination with mechanical and aging properties of the resulting cured coating. Moreover, using a high amount of photoinitiator often results in drawbacks such as a relatively high amount of extractables, yellowing of the cured coating over time, and other undesired properties.
An object of the present invention is to provide a coated optical fiber comprising a cured coating prepared from a liquid curable resin composition which exhibits a fast cure speed without impairing mechanical properties and aging characteristics of the coating. In particular, a coated optical fiber comprising a cured coating having a relatively low yellowing is desired. A further object of the present invention is to provide a radiation curable composition having improved cure speed and a method of increasing the cure speed of a radiation curable composition.